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Physical Review A

Spectral hole burning in the dielectronic recombination from a continuum of finite bandwidth

Wensha Yang, E. S. Shuman, Thomas F. Gallagher

Published Feb 23, 2007• 0 citations
Publisher

Abstract

The presence of an electric field converts the intermediate autoionizing Rydberg $n\ensuremath{\ell}$ states, through which dielectronic recombination passes, into $nk$ Stark states, which have autoionization and capture rates in excess of the radiative decay rates and contribute to dielectronic recombination. In zero field the high $\ensuremath{\ell}$ states do not contribute to dielectronic recombination, but the conversion to Stark states makes it possible and raises the dielectronic recombination rate. However, an electric field can also result in coupling to loss channels which locally reduce the dielectronic recombination rate. We have observed holes in the spectrum of dielectronic recombination from the Ba $6{p}_{3∕2}8g$ continuum of finite bandwidth via the intermediate $6{p}_{1∕2}ng$ states. The holes appear when an electric field is applied, and we attribute them to interaction with rapidly decaying $6{p}_{3∕2}8\ensuremath{\ell}$ states, which diverts flux from dielectronic recombination.

DOI
10.1103/physreva.75.023411

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Enhancement of dielectronic recombination by an electric field

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Dielectronic recombination from a continuum of finite bandwidth has been examined down to very low electric fields, 60 mV/cm, with very high resolution. A tunable laser is used to excite atoms to the continuum of finite bandwidth, the broad $\mathrm{Ba} {6p}_{3/2}11d$ autoionizing state, which straddles the ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit. Atoms which make transitions into the high-lying $\mathrm{Ba} {6p}_{1/2}\mathrm{nd}$ states and radiatively decay to the bound $6snd$ states are detected by field ionization. The recombination, integrated over energy, has been measured for fields from 60 mV/cm to 28 V/cm, showing a peak at 0.5 V/cm, in agreement with expectation based on calculations done for other systems. The final-state distribution has been measured, showing that the outer electron remains a spectator in the radiative decay. At fields below 1.0 V/cm and binding energies less than 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit we find a deviation of our experimental results from the isolated resonance approximation.

Physical Review A•Mar 1, 1999•18 citations
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It should be possible to separate experimentally the contributions to dielectronic recombination (DR) of energetically unresolved intermediate autoionizing Rydberg nlnlnl states using electric fields. This notion is based on two essential ideas. First, electric fields enhance the DR rate by Stark-mixing low-lll states with high autoionization rates with high-lll states with low autoionization rates. Second, the field at which an lll state becomes Stark mixed is determined by its quantum defect, a known function of lll. Consequently, the electric-field dependence of the DR rate should reflect the lll dependence of the autoionization rates and thus the contributions of the zero-field nlnlnl states to the DR rate. This notion cannot be tested experimentally by examining true DR. However, it can be tested by studying DR from a continuum of finite bandwidth (CFB), for in this case the intermediate Rydberg nlnlnl states are restricted to a single value of lll. Specifically, we have examined the electric-field dependence of DR from two CFB's, the Ba6p3∕211d\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{p}_{3∕2}11dBa6p3∕2​11d and 6p3∕28g6{p}_{3∕2}8g6p3∕2​8g states. In these two cases the intermediate autoionizing Rydberg states are restricted to the Ba6p1∕2nd\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{p}_{1∕2}ndBa6p1∕2​nd and 6p1∕2ng6{p}_{1∕2}ng6p1∕2​ng states (l=2and4)(l=2\phantom{\rule{0.5em}{0ex}}\text{and}\phantom{\rule{0.5em}{0ex}}4)(l=2and4), which have quantum defects of 0.25 and 0.02, respectively. For the same nnn they are Stark mixed at fields differing by an order of magnitude. We show experimentally that enhancement of the DR rate occurs at fields differing by a factor of 10 for ndndnd and ngngng states of the same nnn, as expected, confirming that the field dependence of DR can be used to extract information about the contributions of energetically unresolved lll states to the zero-field DR rate.
A small electric field is shown to increase the dielectronic recombination, via autoionizing Rydberg states, of an electron from a continuum of finite bandwidth. The continuum of finite bandwidth is a broad autoionizing state which is part of a series converging to a higher limit, and the field enhancement of the rate occurs because the field converts the nl Rydberg states to nk Stark states, increasing the number of contributing recombination paths. The experimental results are in excellent agreement with the predictions of an isolated resonance approximation treatment and show clearly both the positive effect of Stark mixing and the negative effect of field ionization on dielectronic recombination.
We report the results of a study of the effects of combined electric and magnetic fields on dielectronic recombination (DR) from a continuum of finite bandwidth. Specifically, we have examined the process $\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{p}_{3∕2}8g\ensuremath{\rightarrow}\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{p}_{1∕2}nk\ensuremath{\rightarrow}\mathrm{Ba}\phantom{\rule{0.3em}{0ex}}6{s}_{1∕2}nk+h\ensuremath{\nu}$ in the presence of electric fields from 0to7V∕cm0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}7\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}0to7V∕cm and magnetic fields from 0to250G0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}250\phantom{\rule{0.3em}{0ex}}\mathrm{G}0to250G. Our observations elucidate the requirements for magnetic field enhancement of the DR rate. In particular, they demonstrate that the magnetic coupling must be comparable not only to the electric field splitting of the intermediate autoionizing Rydberg states, but also to their decay rates.
We report the enhancement of dielectronic recombination from a continuum of finite bandwidth in combined electric and magnetic fields. Specifically, we have examined the process Ba $6{p}_{3/2}8g\ensuremath{\rightarrow}6{p}_{1/2}ng\ensuremath{\rightarrow}6{s}_{1/2}ng+h\ensuremath{\nu}$ for $32\ensuremath{\le}n\ensuremath{\le}47$ in combined fields ranging from 0 to 7 V/cm and 0 to 250 G. The results demonstrate that it is the component of $B\ensuremath{\perp}E$ that leads to enhancement. The experimental results indicate that the B\mathit{B}B field coupling must exceed the radiative rate, the autoionization rate, and the Stark splitting of the intermediate autoionizing Rydberg states.
Dielectronic recombination of Ba from a continuum of finite bandwidth in microwave fields of 4, 8, and 12 GHz exhibits a resonant enhancement when the microwave frequency matches a $\ensuremath{\Delta}n=1,$ 2, or 3 transition in the intermediate autoionizing nln\mathcal{l}nl Rydberg states. The origin of the enhancement is resonant Stark mixing of high- and low-l states of different n. Surprisingly, there is little difference between linear and circular polarization of the microwave field, which is attributed to the fact that more than two n levels are coupled in both cases. The dependence of the widths of the resonances on the microwave field is related to how many n states are coupled, and the threshold field for the appearance of the resonances is due to the requirement that low-l states with nonzero quantum defects be mixed with the hydrogenic high-l states. Finally, recombination has been observed from energies in excess of the Ba+{\mathrm{Ba}}^{+}Ba+ 6p1/2{6p}_{1/2}6p1/2​ limit.
Dielectronic recombination from a continuum of finite bandwidth has been examined down to very low electric fields, 60 mV/cm, with very high resolution. A tunable laser is used to excite atoms to the continuum of finite bandwidth, the broad Ba6p3/211d\mathrm{Ba} {6p}_{3/2}11dBa6p3/2​11d autoionizing state, which straddles the Ba+6p1/2{\mathrm{Ba}}^{+} {6p}_{1/2}Ba+6p1/2​ limit. Atoms which make transitions into the high-lying Ba6p1/2nd\mathrm{Ba} {6p}_{1/2}\mathrm{nd}Ba6p1/2​nd states and radiatively decay to the bound 6snd6snd6snd states are detected by field ionization. The recombination, integrated over energy, has been measured for fields from 60 mV/cm to 28 V/cm, showing a peak at 0.5 V/cm, in agreement with expectation based on calculations done for other systems. The final-state distribution has been measured, showing that the outer electron remains a spectator in the radiative decay. At fields below 1.0 V/cm and binding energies less than 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the Ba+6p1/2{\mathrm{Ba}}^{+} {6p}_{1/2}Ba+6p1/2​ limit we find a deviation of our experimental results from the isolated resonance approximation.